Vulcanizates and tire components prepared from compositions including mercapto-functional siloxanes

ABSTRACT

A vulcanizate prepared by a method including introducing an elastomer, a filler, and a mercapto-functional siloxane to form a masterbatch, where the mercapto-functional siloxane is selected from the group consisting of poly(dimethylsiloxane-co-mercaptopropylmethylsiloxane) and dimethoxy mercapto propyl terminated siloxanes, introducing a curative to the masterbatch to form a vulcanizable composition, and subjecting the vulcanizable composition to curing conditions.

This application is a Continuation Application of U.S. application Ser.No. 15/108,022 filed on Jun. 24, 2016, which is a National-StageApplication of PCT/US2014/072511 filed on Dec. 29, 2014, which claimsthe benefit of U.S. Provisional Application Ser. No. 61/921,153 filed onDec. 27, 2013, and are incorporated herein by reference.

FIELD OF THE INVENTION

Embodiments of the present invention are directed toward vulcanizatesand tire components prepared from vulcanizable compositions that areprepared using a mercapto-functional polysiloxane. In one or moreembodiments, it is believed that the mercapto-functional polysiloxanereacts with diene-based rubber within the vulcanizable composition toproduce a polysiloxane-grafted diene-based polymer.

BACKGROUND OF THE INVENTION

Rubber tires employing tire treads have been used for more than onecentury. As the skilled person appreciates, the tire tread provides theinterface between the tire and the road surface and, thus, is importantto the traction performance of the tire. Particularly useful for certainapplications are tire treads with excellent wet traction performance.However, due to numerous complex factors involved, such as thehysteretic bulk deformation of the tread rubber induced by road surfaceasperities, the rate of water drainage between the tread rubber and theroad, lubrication by trapped water or other possible lubricants, and thepossible adhesive interactions between the tread rubber and the road,the quantitative mechanisms attributable to improved wet tractionperformance are not completely understood.

U.S. Pat. No. 6,667,362 teaches rubber compositions having hysteresisproperties at small and large deformations that are comparable to priorart functionalized diene polymers while having advantageous processingproperties in the non-vulcanized state. The rubber compositions includea reinforcing white filler and at least one diene block copolymer whichis intended to interact with said reinforcing white filler, wherein saidcopolymer comprises on at least one end thereof a polysiloxane blockending in a trialkylsilyl group. The diene block copolymer is preparedby reacting a living diene polymer with a polysiloxane block or bysequential polymerization.

SUMMARY OF THE INVENTION

One or more embodiments of this invention provide a vulcanizate preparedby a method comprising introducing an elastomer, a filler, and amercapto-functional siloxane to form a masterbatch and introducing acurative to the masterbatch to form a vulcanizable composition.

Other embodiments of this invention provide a vulcanizable compositioncomprising an elastomer, from about 0.5 to about 20 parts by weight of amercapto-functional siloxane per one hundred parts by weight elastomer,from about 5 to about 200 parts by weight of a filler, and a curativefor the elastomer.

Other embodiments of this invention provide the use of amercapto-functional siloxane in the preparation of a tire tread.

Other embodiments of this invention provide a graft copolymer preparedby introducing a diene-based elastomer and a mercapto-functionalsiloxane to form a mixture and subjecting the mixture to conditions thatwill react the mercapto-functional siloxane to the diene-basedelastomer.

Other embodiments of this invention provide a method for preparing atire, the method comprising introducing an elastomer, a filler, and amercapto-functional siloxane to form a masterbatch, introducing acurative to the masterbatch to form a vulcanizable composition, formingthe vulcanizable composition into a green tire tread, building a greentire by using the green tire tread as the tire tread component of thegreen tire, and subjecting the green tire to curing conditions to form atire.

Other embodiments of this invention provide a vulcanizable compositionof matter comprising an elastomer, a graft copolymer prepared byreacting a mercapto-functional siloxane with a diene-based elastomer, afiller, and a curative.

DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

Embodiments of the invention are based, at least in part, on thediscovery of a rubber vulcanizate that is prepared from a vulcanizablecomposition that includes a mercapto-functional siloxane polymer. Thevulcanizate of one or more embodiments advantageously demonstrates oneor more of relatively low hysteretic loss, relatively high wet skidresistance, and relatively low bleed of siloxane polymer from thevulcanizate. As a result, the vulcanizates of this invention areadvantageously useful as tire treads.

Vulcanizable Composition

In one or more embodiments, the vulcanizable compositions of matter ofthis invention are prepared by combining an elastomer, a filler, acurative, and a mercapto-functional polysiloxane. Other optionalingredients may include those ingredients that are included invulcanizable compositions of this nature including, but not limited to,cure activators, cure accelerators, oils, resins, plasticizers,pigments, fatty acids, zinc oxide, and peptizing agents. In one or moreof these embodiments, it is believed that the mercapto-functionalpolysiloxane reacts with the elastomer (e.g. a diene-based elastomer) toform a graft copolymer. In one or more embodiments, this reaction isbelieved to take place in situ during solid-state mixing of theelastomer and the mercapto-functional polysiloxane at one or moreappropriate conditions that lead to graft copolymer formation.

In other embodiments, the graft copolymer is pre-formed by reacting adiene-based elastomer with the mercapto-functional polysiloxane. Thisgraft copolymer may then be employed to form a vulcanizable compositionof matter by combining this graft copolymer with optional additionalelastomer, filler, and curative.

Rubber

In one or more embodiments, the elastomer employed to prepare thevulcanizable compositions of this invention may include those polymersthat can be vulcanized to form compositions possessing rubbery orelastomeric properties. These elastomers may include natural andsynthetic rubbers. These elastomers may include those that react withthe mercapto-functional siloxanes or they may include those otherwiseadded to and present with the vulcanizable composition of matter. Inother words, the vulcanizable compositions may include elastomers otherthat those that react with the mercapto-functional siloxanes.

In one or more embodiments, reference may be made to diene-basedelastomers, which include those synthetic rubbers that derive from thepolymerization of conjugated diene monomer or the copolymerization ofconjugated diene monomer with one or more comonomer such asvinyl-substituted aromatic monomer. For purposes of this specification,elastomers deriving from the polymerization of conjugated dienes may bereferred to as diene-based elastomers. Conjugated diene monomersinclude, but are not limited to, 1,3-butadiene, isoprene,1,3-pentadiene, 1,3-hexadiene, 2,3-dimethyl-1,3-butadiene,2-ethyl-1,3-butadiene, 2-methyl-1,3-pentadiene, 3-methyl-1,3-pentadiene,4-methyl-1,3-pentadiene, 2,4-hexadiene, and combinations thereof. Assuggested above, the diene-based elastomer can include copolymers wherethe conjugated diene is copolymerized with comonomer such as, but notlimited to, vinyl aromatic monomer such as styrene, α-methylstyrene,p-methylstyrene, o-methylstyrene, p-butylstyrene, vinylnaphthalene, andcombinations thereof.

Exemplary elastomers include natural rubber, synthetic polyisoprene,polybutadiene, polyisobutylene-co-isoprene, neoprene,poly(ethylene-co-propylene), poly(styrene-co-butadiene),poly(styrene-co-isoprene), poly(styrene-co-isoprene-co-butadiene),poly(isoprene-co-butadiene), poly(ethylene-co-propylene-co-diene),polysulfide rubber, acrylic rubber, urethane rubber, silicone rubber,epichlorohydrin rubber, and mixtures thereof. These elastomers can havea myriad of macromolecular structures including linear, branched, andstar-shaped structures. These elastomers may also include one or morefunctional units, which typically include heteroatoms.

In one or more embodiments, useful elastomers include high molecularweight polymers such as those having a number average molecular weightin excess of 50 kg/mol, in other embodiments in excess of 100 kg/mol, inother embodiments in excess of 125 kg/mol, and in other embodiments inexcess of 150 kg/mol.

Filler

The filler may include one or more conventional reinforcing ornon-reinforcing fillers. For example, useful fillers include carbonblack, silica, alumina, and silicates such as calcium, aluminum, andmagnesium silicates. In particular embodiments, the vulcanizablecompositions of this invention include a silica filler used incombination with a silica coupling agent.

In one or more embodiments, carbon blacks include furnace blacks,channel blacks, and lamp blacks. More specific examples of carbon blacksinclude super abrasion furnace (SAF) blacks, intermediate super abrasionfurnace (ISAF) blacks, high abrasion furnace (HAF) blacks, fastextrusion furnace (FEF) blacks, fine furnace (FF) blacks,semi-reinforcing furnace (SRF) blacks, medium processing channel blacks,hard processing channel blacks, conducting channel blacks, and acetyleneblacks. Representative carbon blacks useful in one or more embodimentsmay include those designated by ASTM D1765 as N326, N330, N339, N343,N347, N351, N358, N550, N650, N660, N762, N772, and N774.

In particular embodiments, the carbon blacks may have a surface area(EMSA) of at least 20 m²/g, in other embodiments at least 35 m²/g, inother embodiments at least 50 m²/g, in other embodiments at least 60m²/g; surface area values can be determined by ASTM D-1765 using thecetyltrimethylammonium bromide (CTAS) technique. In particularembodiments, the sidewalls include carbon black filler having a surfacearea (EMSA) of from about 60 to about 110 m²/g. The carbon blacks may bein a pelletized form or an unpelletized flocculent form. The preferredform of carbon black may depend upon the type of mixing equipment usedto mix the rubber compound. Exemplary carbon blacks that are useful inthe practice of this invention include those characterized by ASTMD-1765, such as N-110, N-220, N-339, N-330, N-351, N-550, N-660, andN990 grades.

In one or more embodiments, the filler may include silica. When silicais used as a filler, the silica may be employed in conjunction with acoupling agent. In these or other embodiments, the silica may be used inconjunction with a silica dispersing agent.

In one or more embodiments, useful silicas include, but are not limitedto, precipitated amorphous silica, wet silica (hydrated silicic acid),dry silica (anhydrous silicic acid), fumed silica, calcium silicate, andthe like. Other suitable fillers include aluminum silicate, magnesiumsilicate, and the like. In particular embodiments, the silica is aprecipitated amorphous wet-processed hydrated silica. In one or moreembodiments, these silicas are produced by a chemical reaction in water,from which they are precipitated as ultra-fine, spherical particles.These primary particles are believed to strongly associate intoaggregates, which in turn combine less strongly into agglomerates.

Some commercially available silicas that may be used include Hi-Sil™215, Hi-Sil™ 233, and Hi-Sil™ 190 (PPG Industries, Inc.; Pittsburgh,Pa.). Other suppliers of commercially available silica include GraceDavison (Baltimore, Md.), Degussa Corp. (Parsippany, N.J.), RhodiaSilica Systems (Cranbury, N.J.), and J.M. Huber Corp. (Edison, N.J.).

In one or more embodiments, silicas may be characterized by theirsurface areas, which give a measure of their reinforcing character. TheBrunauer, Emmet and Teller (“BET”) method (described in J. Am. Chem.Soc., vol. 60, p. 309 et seq.) is a recognized method for determiningthe surface area. The BET surface area of silica is generally less than450 m²/g. Useful ranges of surface area include from about 32 to about400 m²/g, about 100 to about 250 m²/g, and about 150 to about 220 m²/g.

In one or more embodiments, the pH of silica may be from about 5 toabout 7 or slightly over 7, or in other embodiments from about 5.5 toabout 6.8.

In one or more embodiments, useful silica coupling agents includesulfur-containing silica coupling agents. Examples of sulfur-containingsilica coupling agents include bis(trialkoxysilylorgano)polysulfides ormercapto-organoalkoxysilanes. Types ofbis(trialkoxysilylorgano)polysulfides includebis(trialkoxysilylorgano)disulfide andbis(trialkoxysilylorgano)tetrasulfides. Exemplary silica dispersing aidsinclude, but are not limited to an alkyl alkoxysilane, a fatty acidester of a hydrogenated or non-hydrogenated C₅ or C₆ sugar, apolyoxyethylene derivative of a fatty acid ester of a hydrogenated ornon-hydrogenated C₅ or C₆ sugar, and mixtures thereof, or a mineral ornon-mineral additional filler.

Still other useful fillers that may be used, especially in conjunctionwith silica and/or carbon black include, but are not limited to, mineralfillers such as clay (e.g. hydrous aluminum silicate), talc (hydrousmagnesium silicate), aluminum hydrate (Al(OH)₃), and mica; as well asmetal oxides such as aluminum oxide; and the like. Additional usefulfillers suitable for use in the rubber compositions disclosed herein areknown to those skilled in the art CURATIVE

A multitude of rubber curing agents (also called vulcanizing agents) maybe employed, including sulfur or peroxide-based curing systems. Curingagents are described in Kirk-Othmer, ENCYCLOPEDIA OF CHEMICALTECHNOLOGY, Vol. 20, pgs. 365-468, (3^(rd) Ed. 1982), particularlyVulcanization Agents and Auxiliary Materials, pgs. 390-402, and A. Y.Coran, Vulcanization, ENCYCLOPEDIA OF POLYMER SCIENCE AND ENGINEERING,(2′ Ed. 1989), which are incorporated herein by reference. In one ormore embodiments, the curative is sulfur. Examples of suitable sulfurvulcanizing agents include “rubberrmaker's” soluble sulfur; sulfurdonating vulcanizing agents, such as an amine disulfide, polymericpolysulfide or sulfur olefin adducts; and insoluble polymeric sulfur.Vulcanizing agents may be used alone or in combination.

In one or more embodiments, the curative is employed in combination witha cure accelerator. In one or more embodiments, accelerators are used tocontrol the time and/or temperature required for vulcanization and toimprove properties of the vulcanizate. Examples of accelerators includethiazol vulcanization accelerators, such as 2-mercaptobenzothiazol,dibenzothiazyl disulfide, N-cyclohexyl-2-benzothiazyl-sulfenamide (CBS),and the like, and guanidine vulcanization accelerators, such asdiphenylguanidine (DPG) and the like.

Mercapto-Functional Polysiloxanes

In one or more embodiments, the mercapto-functional polysiloxanes, whichmay also be referred to as mercapto-modified siloxanes,mercapto-modified polysiloxanes, or mercapto-modified silicones, includecopolymers that include at least one of the each of the following units:

where R¹, R², and R³ are each independently a monovalent organic groupand each R⁴ is a divalent organic group.

In particular embodiments, the mercapto-modified polysiloxanes may bedefined by the formula

where R¹, R², and R³ are each independently a monovalent organic group,each R⁴ is a divalent organic group, and m and n are integers and theratio of m to (m+n) is from about 0.03 to about 0.08.

In one or more embodiments, the monovalent organic group includeshydrocarbyl groups such as but not limited to alkyl, cycloalkyl,substituted cycloalkyl, alkenyl, cycloalkenyl, substituted cycloalkenyl,aryl, allyl, substituted aryl, aralkyl, alkaryl, and alkynyl groups,with each group preferably containing from 1 carbon atom, or theappropriate minimum number of carbon atoms to form the group, up to 20carbon atoms. These hydrocarbyl groups may contain heteroatoms such as,but not limited to, nitrogen, oxygen, silicon, sulfur, and phosphorusatoms.

The divalent organic group includes a hydrocarbylene group orsubstituted hydrocarbylene group such as, but not limited to, alkylene,cycloalkylene, substituted alkylene, substituted cycloalkylene,alkenylene, cycloalkenylene, substituted cycloalkenylene, substitutedcycloalkenylene, arylene, and substituted arylene groups, with eachgroup preferably containing from 1 carbon atom, or the appropriateminimum number of carbon atoms to form the group, up to about 20 carbonatoms. A substituted hydrocarbylene group is a hydrocarbylene group inwhich one or more hydrogen atoms have been replaced by a substituentsuch as an alkyl group. The divalent organic groups may also contain oneor more heteroatoms such as, but not limited to, nitrogen, oxygen,boron, silicon, sulfur and phosphorous atoms.

In one or more embodiments, suitable mercapto-functionalizedpolysiloxanes includepoly(dimethylsiloxane-co-mercaptopropylmethylsiloxane). In otherembodiments, useful mercapto-functionalized siloxanes include dimethoxymercapto propyl terminated siloxanes.

In one or more embodiments, useful mercapto-functional polysiloxanes areknown in the art as described in U.S. Pat. Publ. Nos. 2009/0126845 and2010/0284957, which are incorporated herein by reference. Usefulmercapto-functional siloxanes can be purchased under the tradenamesKF-2001 and KF-2004 from Shin-Etsu Chemical Co., Ltd.; SMS-022, SMS-042and SMS-992 from Gelest, Inc.; and PS848, PS849, PS849.5, PS850, PS850.5and PS927 from United Chemical Technologies. These commercial productseach differ with respect to weight-average molecular weight, molecularweight distribution, and mercapto group ratio, and they can be selectedas desired.

In one or more embodiments, the average number of repeating monomerunits with the mercapto-modified siloxane is from about 90 to about 410,in other embodiments from about 110 to about 350, and in otherembodiments from about 130 to about 300. In these or other embodiments,the molecular weight of the mercapto-modified siloxane is from about5,000 about 50,000 g/mol, in other embodiments from about 6,800 to about30,000 g/mol, and in other embodiments from about 7,500 to about 25,000g/mol.

In one or more embodiments, after polymerization and cross-linking,solid siloxane samples will present an external hydrophobic surface.This surface chemistry may make it difficult for polar solvents (such aswater) to wet the siloxane surface, and may lead to adsorption ofhydrophobic contaminants. Plasma oxidation can be used to alter thesurface chemistry, adding silanol (SiOH) groups to the surface. Thistreatment renders the siloxane surface hydrophilic, allowing water towet the surface. The oxidized surface resists adsorption of hydrophobicand negatively charged species. The oxidized surface can be furtherfunctionalized by reaction with trichlorosilanes. Oxidized surfaces arestable for ˜30 minutes in air, after a certain time hydrophobic recoveryof the surface is inevitable independently of the surrounding mediumwhether it is vacuum, air, or water.

In one or more embodiments, silane precursors with more acid forminggroups and fewer methyl groups, such as methyltrichlorosilane, can beused to introduce branches or cross links in the polymer chain. Underideal conditions, each molecule of such a compound becomes a branchpoint. This can be used to produce hard silicone resins. In a similarmanner, precursors with three methyl groups can be used to limitmolecular weight, since each such molecule has only one reactive siteand so forms the end of a siloxane chain.

In one or more embodiments, the siloxane polymer is manufactured inmultiple viscosities, ranging from a thin pourable liquid (when n isvery low), to a thick rubbery semi-solid (when n is very high). Thesiloxane molecules have quite flexible polymer backbones (or chains) dueto their siloxane linkages. In one or more embodiments, these flexiblechains become loosely entangled when molecular weight is high, which mayresult in siloxanes with unusually high level of viscoelasticity, andthe loss tangent is very low (tan 6<<0.001).

Other Ingredients

Other ingredients that are typically employed in rubber compounding mayalso be added to the rubber compositions. These include oils,plasticizer, waxes, scorch inhibiting agents, processing aids, zincoxide, tackifying resins, reinforcing resins, fatty acids such asstearic acid, peptizers and antidegradants such as antioxidants,antiozonants, and waxes. In particular embodiments, the oils that areemployed include those conventionally used as extender oils, which aredescribed above. Useful oils or extenders that may be employed include,but are not limited to, aromatic oils, paraffinic oils, naphthenic oils,vegetable oils other than castor oils, low PCA oils including MES, TDAE,and SRAE, and heavy naphthenic oils.

Ingredient Amounts Rubber

In one or more embodiments, the vulcanizable compositions include atleast 20, in other embodiments at least 30, and in other embodiments atleast 40 percent by weight of the rubber component, based upon theentire weight of the composition. In these or other embodiments, thevulcanizable compositions include at most 90, in other embodiments atmost 70, and in other embodiments at most 60 percent by weight of therubber component based on the entire weight of the composition. In oneor more embodiments, the vulcanizable compositions include from about 20to about 90, in other embodiments from about 30 to about 70, and inother embodiments from about 40 to about 60 percent by weight of therubber component based upon the entire weight of the composition.

Filler

In one or more embodiments, the vulcanizable compositions include atleast 5, in other embodiments at least 25, and in other embodiments atleast 40 parts by weight (pbw) filler (e.g. silica) per 100 parts byweight rubber (phr). In these or other embodiments, the vulcanizablecomposition includes at most 200, in other embodiments at most 120, andin other embodiments at most 70 pbw of the filler phr. In one or moreembodiments, the vulcanizable composition includes from about 5 to about200, in other embodiments from about 10 to about 100, in otherembodiments from about 25 to about 120, and in other embodiments fromabout 40 to about 70 pbw of filler phr.

Mercapto-Functional Siloxane

In one or more embodiments, the vulcanizable compositions include atleast 0.5, in other embodiments at least 2.0, in other embodiments atleast 3.0, in other embodiments at least 4.0, and in other embodimentsat least 5.0 parts by weight (pbw) mercapto-functional siloxane per 100parts by weight rubber (phr). In these or other embodiments, thevulcanizable composition includes at most 20, in other embodiments atmost 15, in other embodiments at most 12, in other embodiments at most10 pbw mercapto-functional siloxane phr. In one or more embodiments, thevulcanizable composition includes from about 0.5 to about 20, in otherembodiments from about 3.0 to about 12, and in other embodiments fromabout 5.0 to about 10 pbw mercapto-functional siloxane phr. In one ormore embodiments, reference to the amount of mercapto-functionalsiloxane refers to the unreacted mercapto-functional siloxane as itexists prior to any reaction with a diene-based elastomer. Inasmuch asthe weight of the reacted mercapto-functional siloxane will notappreciably change upon reaction (e.g. a grafting reaction) with adiene-based elastomer, reference may also be made to the weight of themercapto-functional siloxane residue of the graft copolymer (i.e. thesiloxane portion of the graft copolymer formed by reacting themercapto-functional siloxane with a diene-based elastomer.

Cure System

The skilled person will be able to readily select the amount ofvulcanizing agents to achieve the level of desired cure. Also, theskilled person will be able to readily select the amount of cureaccelerators to achieve the level of desired cure.

Mixing Procedure

All ingredients of the vulcanizable compositions can be mixed withstandard mixing equipment such as Banbury or Brabender mixers,extruders, kneaders, and two-rolled mills. In one or more embodiments,this may include a multi-stage mixing procedure where the ingredientsare mixed in two or more stages. For example, in a first stage (which isoften referred to as a masterbatch mixing stage), the elastomer, fillerand optionally the mercapto-functional siloxane is mixed. This mixing,which takes place in the absence of the curative, can proceed attemperature above which the curing would otherwise take place if thecurative was present. For example, this mixing can take place attemperatures in excess of 120° C., in other embodiments in excess of130° C., in other embodiments in excess of 140° C., and in otherembodiments in excess of 150° C. In one or more embodiments, it isbelieved that these conditions are sufficient to affect a reactionbetween the diene-based elastomer and the mercapto-functional siloxane.

Once the masterbatch is prepared, the vulcanizing agents may beintroduced and mixed into the masterbatch in a final mixing stage, whichis typically conducted at relatively low temperatures so as to reducethe chances of premature vulcanization. For example, this mixing maytake place at temperatures below 120° C., in other embodiments below110° C., in other embodiments below 100° C. Additional mixing stages,sometimes called remills, can be employed between the masterbatch mixingstage and the final mixing stage.

As suggested above, in one or more embodiments, the mercapto-functionalsiloxane reacts with a diene-based elastomer to form a graft copolymerwhere the siloxane polymer extends from the diene-based elastomer as agraft. Without wishing to be bound by any particular theory, it isbelieved that the sulfur functionality of the mercapto-functionalsiloxane reacts with unsaturation along the backbone of the diene-basedelastomer to thereby form a covalent bond that results in siloxanegrafts at one or more locations along the backbone of the diene-basedelastomer. Since the reaction is believed to take place at theunsaturation within the backbone of the diene-based polymer, thediene-based polymer need not otherwise be reactive. For example, in oneor more embodiments, the reaction between the mercapto-functionalpolymer and the diene-based elastomer takes place while the diene-basedelastomer is non-living. In accordance with certain embodiments, themercapto-functional polydimethylsiloxane copolymer reacts along theunsaturated diene polymer chain at a point at least about 1000 g/mol ofpolymer units from the terminus of the polymer chain. Alternatively,about 2000 g/mol of polymer units from the terminus of the polymerchain, or up to about 3000 g/mol remaining from the terminal of thepolymer chain.

As also suggested above, where a reaction takes place between themercapto-functional siloxane and the diene-based elastomer, the reactioncan take place in situ, which refers to a reaction taking place in thepresence of at least one additional ingredient of the vulcanizablecomposition, such as the filler. This in situ reaction can take placeduring formation of the masterbatch (i.e. it takes place duringsolid-state mixing). In other embodiments, the reaction can take placeafter formation of the masterbatch, such as in a remill. For example,the elastomer and the filler can be mixed, and then themercapto-functional elastomer can be added in a subsequent mixing step,such as remill. Mixing may continue either at elevated temperatures,such as those used to prepare the masterbatch, or remill mixing may takeplace at lower temperatures.

In yet other embodiments, the graft copolymer can be prepared in advanceof the formation of the vulcanizable composition. For example, themercapto-functional siloxane and a diene-based elastomer may be mixedexclusive of other ingredients of the vulcanizable composition atconditions, such as temperature, sufficient to graft themercapto-function siloxane to the diene-based elastomer. The resultingmixture, which may include a grafted copolymer, can then be employed inthe formation of the vulcanizable compositions of this invention.

Preparation of Tire

The vulcanizable compositions of this invention can be processed intotire components according to ordinary tire manufacturing techniquesincluding standard rubber shaping, molding and curing techniques.Typically, vulcanization is effected by heating the vulcanizablecomposition in a mold; e.g., it may be heated to about 140° C. to about180° C. Cured or crosslinked rubber compositions may be referred to asvulcanizates, which generally contain three-dimensional polymericnetworks that are thermoset. The other ingredients, such as fillers andprocessing aids, may be evenly dispersed throughout the crosslinkednetwork. In particular embodiments, one or more of the compoundingredients, such as the mercapto-functional siloxane or a graftcopolymer resulting from a reaction between the mercapto-functionalsiloxane and a diene-based copolymer, may become crosslinked orotherwise chemically bonded to the crosslinked rubber network. As theskilled person will appreciate, the amounts of the various ingredients,especially those that do not react, will remain within the cured tirecomponent the same as they existed within the compound.

In one or more embodiments, the vulcanizable composition of matter ofthe present invention are particularly useful for making tire treads.Pneumatic tires can be made as discussed in U.S. Pat. Nos. 5,866,171,5,876,527, 5,931,211, and 5,971,046, which are incorporated herein byreference. For example, the various tire components can be prepared asgreen tire components (i.e., uncured tire components), and assembledinto a green tire. The green tire can then be subjected to curingconditions to form a vulcanized tire wherein the various greencomponents are generally adhered to one another through thevulcanization process. Depending upon the ultimate use for the rubbercomposition, it may be processed (e.g. milled) into sheets prior tobeing formed into any of a variety of components and then vulcanized,which typically occurs at about 5° C. to about 15° C. higher than thehighest temperatures employed during the mixing stages, most commonlyabout 170° C.

Characteristics of Vulcanizate

In one or more embodiments, the vulcanizates of the present inventionare characterized by an advantageous balance of properties. Inparticular embodiments, the vulcanizates are characterized by anadvantageous balance between hysteretic loss, wet skid resistance, andlow loss of oil, such as polysiloxane, through bleeding.

In one or more embodiments, the vulcanizates of the present inventionare characterized by hydrophilic/hydrophobic properties. Theseproperties are believed to contribute to enhanced the wet skidresistance of the vulcanizate.

In these or other embodiments, the vulcanizates of the present inventionare characterized by advantageous hysteretic loss properties, which isindicative of lower rolling resistance for tires including one or morecomponents prepared from the compositions of this invention. In one ormore embodiments, the vulcanizable composition of this inventiontherefore product vulcanizates characterized by not only in improved wettraction, but also lowers rolling resistance.

In accordance with certain embodiments, the surfaces of the vulcanizedrubbers prepared from the rubber compositions disclosed herein exhibitan adjusted relative hydrophobicity or hydrophilicity as compared to thesurfaces of vulcanized rubbers formed from rubber compositions preparedusing the same formulation, but using conventional polymers instead ofthe mercapto-functional polydimethylsiloxane grafted polymer.

The adjustment of the relative hydrophobicity or hydrophilicity of therubber surface can contribute to the enhancement of the wet tractionperformance of a tire tread made with the rubber compositions disclosedherein. As mentioned above, due to numerous complex factors involved,the quantitative mechanisms attributable to improved wet tractionperformance are not completely understood.

However, in combination with other of the complex factors involved, theadjusted relative hydrophobicity or hydrophilicity can act to enhancethe wet traction performance, particularly, the wet skid resistance of atire tread made from the rubber compositions disclosed herein. Forexample, tire treads having surfaces that are hydrophobic will tend torepulse the water at the tread surface and will likely facilitate thewater drainage from between the tire tread's surface and the roadsurface.

Conversely, tire treads that have a hydrophilic surface will tend toattract water and are more likely to form “adhesive” capillary bridgesbetween the tire tread's surface and the road surface. Thus, byadjusting the relative hydrophobicity or relative hydrophilicity ascompared to a vulcanized rubber made from the same composition but withconventional grafted polymer instead of the mercapto-functionalpolydimethylsiloxane grafted polymer, the rubber compositions disclosedherein can contribute to the enhancement of the wet skid resistance ofthe tire tread.

Moreover, the use of the mercapto-functional polydimethylsiloxanegrafted polymer does not significantly affect certain important bulkmechanical properties of such rubber, including but not limited toproperties directed to dynamic viscoelasticity and tensile strength.

Still further, the vulcanizates of one or more embodimentsadvantageously exhibit relatively low bleed of low molecular weightpolymers, such as polysiloxanes, to the surface of the rubbercompositions.

The following examples are for purposes of illustration only and are notintended to limit the scope of the claims which are appended hereto.

EXAMPLES

TABLE 1 Properties of mercapto-functionalized polydimethylsiloxane Vis-Molecular Mole % cosity Weight Mercapto Polymer Supplier (CPs) (g/mol)Monomer Copolymer 1 SMS-022 Gelest, Inc. 120-180 6000-8000 2-3 Copolymer2 PS849 United 100-200 Not 20-25 Chemical reported Technology

TABLE 2 Formation and compounding properties of rubber stocksComparative Example 1 Example 1 Example 2 Compound (phr) SBR 80 80 80 NR20 20 20 Silica 55 55 55 Silane coupling agent 5 5 5 Copolymer 1 — 10 —Copolymer 2 — — 10 Black Oil 10 10 10 Stearic Acid 2 2 2 Wax 2 2 2Antioxidant 0.95 0.95 0.95 Sulfur 1.5 1.5 1.5 Accelerator 0.7 0.7 0.7Accelerator 2 2 2 Accelerator 1.4 1.4 1.4 Zinc Oxide 2.5 2.5 2.5Properties Rolling resistance index 100 103 111 Wet skid resistanceindex 100 105 105

The characteristics of the mercapto-functionalized polydimethylsiloxanecopolymers are shown in Table 1. The ingredients employed in each Sampleare presented in Table 2. Each rubber compound was prepared in threestages named initial, remill and final. In the initial mix, SBR, NR,silica, antioxidant, stearic acid, oil and optionally themercapto-functionalized siloxane copolymer were mixed.

The initial portion of the compound was mixed in a 65 g Banbury mixeroperating at 50 RPM and 133° C. First, polymer was placed in the mixer,and after 0.5 minutes, the remaining ingredients except the stearic acidwere added. The stearic acid was then added after 3 minutes. The initialstages were mixed for 5 to 6 minutes. At the end of mixing thetemperature was approximately 165° C. The sample was transferred to amill operating at a temperature of 60° C., where it was sheeted andsubsequently cooled to room temperature.

The remill was mixed by adding the resulting initial mixture and silaneshielding agent to the mixer simultaneously. The initial mixertemperature was 95° C. and it was operating at 50 RPM. The finalmaterial was removed from the mixer after three minutes when thematerial temperature was 150° C. The sample was transferred to a milloperating at a temperature of 60° C., where it was sheeted andsubsequently cooled to room temperature.

The final stage was mixed by adding the remill mixture and the curativematerials to the mixer simultaneously. The initial mixer temperature was65° C. and it was operating at 45 RPM. The final material was removedfrom the mixer after 2.5 minutes when the material temperature wasbetween 95 to 105° C. The final mixtures were sheeted into buttons andbars for testing wet traction. The samples were cured at 171° C. for 15minutes in standard molds placed in a hot press.

For evaluation of a vulcanized rubber, wet skid resistance indexmeasured according to ASTM E303-83 using a portable skid tester made byStanley Inc. The wet skid number was indexed to Comparative Example 1.

Rolling resistance index was measured as the tan delta value of eachcompound using an ARES viscoelasticity tester made by TAInstruments—Water LLC under the following conditions: 50° C., 15 Hzfrequency and 10% dynamic strain. The rolling resistance was indexed toComparative Example 1. The larger the index value, the lower the rollingresistance is. The rolling resistance index and wet skid resistanceindex are shown in Table 2.

As shown in Table 1, Examples 1 and 2 were prepared using themercapto-functionalized polydimethylsiloxane copolymers identified asCopolymer 1 and 2. Comparative Example 1 did not include amercapto-functionalized polydimethylsiloxane copolymer.

The results provided in Table 2 show that the use ofmercapto-functionalized polydimethylsiloxane copolymers (Examples 1 and2) adjusts the relative hydrophobicity or hydrophilicity of thevulcanizate as compared to the control vulcanizate of ComparativeExample 1 prepared, including no mercapto-functionalizedpolydimethylsiloxane copolymer.

Similar experiments were conducted using polysiloxanes that were notmercapto-functionalized; i.e. the polysiloxanes did not include amercapto-functional group. Vulcanizable compositions including thesenon-functional siloxanes were vulcanized, and upon aging, it wasobserved that oils, which were believed to include polysiloxane oils,appreciably bled from the vulcanizate. This level of oil bleed wassignificant and appreciable compared to those samples wherein amercapto-functional siloxane was employed.

Various modifications and alterations that do not depart from the scopeand spirit of this invention will become apparent to those skilled inthe art. This invention is not to be duly limited to the illustrativeembodiments set forth herein.

What is claimed is:
 1. A vulcanizate prepared by a method comprising: a.introducing an elastomer, a filler, and a mercapto-functional siloxaneto form a masterbatch, where the mercapto-functional siloxane isselected from the group consisting ofpoly(dimethylsiloxane-co-mercaptopropylmethylsiloxane) and dimethoxymercapto propyl terminated siloxanes; b. introducing a curative to themasterbatch to form a vulcanizable composition; and c. subjecting thevulcanizable composition to curing conditions.
 2. The vulcanizate ofclaim 1, where the elastomer is a diene-based elastomer.
 3. Thevulcanizate of claim 1, where the filler includes silica.
 4. Thevulcanizate of claim 1, where said step of introducing introduces fromabout 5 to about 200 parts by weight filler per one hundred parts byweight elastomer and from about 0.5 to about 20 parts by weightmercapto-functional siloxane per one hundred parts by weight elastomer.5. The vulcanizate of any of claim 1, where the vulcanizate is a tiretread.
 6. A vulcanizable composition comprising: a. an elastomer; b.from about 0.5 to about 20 parts by weight of a mercapto-functionalsiloxane per one hundred parts by weight elastomer, where themercapto-functional siloxane is selected frompoly(dimethylsiloxane-co-mercaptopropylmethylsiloxane) and dimethoxymercapto propyl terminated siloxanes; c. from about 5 to about 200 partsby weight of a filler; and d. a curative for the elastomer.
 7. Thevulcanizable composition of claim 6, where the mercapto-functionalsiloxane is a distinct molecular species within the vulcanizablecomposition.
 8. The vulcanizable composition of claim 6, where themercapto-functional siloxane is covalently bonded at least one of saidelastomer.
 9. The vulcanizable composition of claim 6, where theelastomer is a diene-based elastomer.
 10. The vulcanizable compositionof claim 6, where the filler includes silica.
 11. A vulcanizablecomposition comprising: a. an elastomer; b. from about 0.5 to about 20parts by weight of a mercapto-functional siloxane per one hundred partsby weight elastomer, where the mercapto-functional siloxane consists ofthe following repeat units:

where R¹, R², and R³ are each independently a monovalent organic groupand each R⁴ is a divalent organic group; c. from about 5 to about 200parts by weight of a filler; and d. a curative for the elastomer. 12.The vulcanizable composition of claim 11, where the elastomer is adiene-based elastomer.
 13. The vulcanizable composition of claim 11,where the filler includes silica.